Conventionally, so-called exhaust gas recirculation (EGR) has been conducted in, for example, a vehicle engine in which part of exhaust gas is extracted from an exhaust side and is returned to an intake side. The exhaust gas returned to the intake side suppresses combustion of fuel in the engine, thereby lowering combustion temperature to reduce NOx generated.
Generally, in order to conduct such kind of exhaust gas recirculation, an EGR pipe connects an appropriate position in an exhaust passage, extending from an exhaust manifold to an exhaust pipe, with an appropriate position in an intake passage, extending from an intake pipe to an intake manifold, whereby the exhaust gas is recirculated through the EGR pipe.
The EGR pipe may be equipped with, midway thereof, a water-cooled EGR cooler since cooling, in a mid part of the EGR pipe, of the exhaust gas to be recirculated to the engine will drop a temperature of and reduce a volume of the exhaust gas to lower a combustion temperature without substantial decrease in output of the engine, thereby effectively suppressing generation of nitrogen oxides.
There is, for example, a fear in the EGR cooler that a heat-exchanger tube may be clogged by deposition of soot entrained in the exhaust gas. Such clog may result in failed recirculation of the exhaust gas in an intended amount, leading to unattainment of a target EGR ratio. Such abnormal situation of the unattained target EGR ratio is required to determine by any way.
There exists, for example, the following Patent Literature 1 as a prior art literature pertinent to the invention.